It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid developers comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0, and a high vapor pressure. The toner particles are less than 30 .mu.m average particle size as determined using the Malvern Particle Sizer described below. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably adjuvants, e.g., polyhydroxy compounds, polybutylene succinimide, an aromatic hydrocarbon, etc., to the liquid developer comprising the thermoplastic resin, nonpolar liquid, and preferably a colorant. Such liquid developers provide images of good resolution, but it has been found that charging and image quality are particularly pigment dependent. Some formulations, suffer from poor image quality manifested by low resolution, poor solid area coverage (density), and/or image squash. Some formulations result in wrong sign (negative) developers. In order to overcome such problems much research effort has been expended to develop new type charge directors and/or charging adjuvants for electrostatic liquid developers.
It has been found that the above disadvantages can be overcome and improved positive-charged developers prepared containing a nonpolar liquid, ionic or zwitterionic charge director compound, a thermoplastic resin, and preferably a colorant and a hydrocarbon soluble mineral acid adjuvant as described below. The electrostatic liquid developer when used to develop an electrostatic image results in improved image quality, reduced squash, improved solid area coverage independent of the pigment and charge director compound present.